U.S. Rep. Tammy Duckworth, who was born in Thailand to a Thai mother of Chinese descent and an American father, is an Iraq War veteran. Kirk apologized for the comment on Twitter

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Republican U.S. Sen. Mark Kirk, right, and Democratic U.S. Rep. Tammy Duckworth, left, face off in their first televised debate in what's considered a crucial race that could determine which party controls the Senate, Oct. 27, 2016, at the University of Illinois in Springfield, Ill.

Illinois Republican U.S. Sen. Mark Kirk apologized Friday after he mocked his Democratic rival's family history of military service by saying he had forgotten the Asian-American congresswoman's "parents came all the way from Thailand to serve George Washington."

The comment came Thursday evening during the first televised debate between Kirk and U.S. Rep. Tammy Duckworth, at the University of Illinois in Springfield. The Illinois Senate contest is one of a handful of races that could determine which party controls the chamber next year.

Duckworth, an Iraq War veteran who lost both legs when the Black Hawk she was piloting was shot down in 2004, was born in Bangkok. Her mother is Thai and she has said her father first went to Southeast Asia while serving with the Marines in Vietnam.

The congresswoman said her family has "served this nation in uniform going back to the Revolution."

Kirk's campaign said on Twitter Friday, "Sincere apologies to an American hero, Tammy Duckworth, and gratitude for her family's service."

The apology followed an earlier statement by the campaign that said the senator had "consistently called Rep. Duckworth a war hero and honors her family’s service to this country." But that statement also went on to attack Duckworth on other issues.

Duckworth, the first Asian-American congresswoman from Illinois, later tweeted a photo of herself with her parents, including her father in uniform displaying his medals. Her tweet says: "My mom is an immigrant and my dad and his family have served this nation in uniform since the Revolution."

Kirk, who suffered a stroke in 2012 and returned to work one year later, has been criticized before for controversial statements. In August, he said President Barack Obama was acting like the "drug dealer in chief" when the U.S. made a $400 million payment to Iran contingent on the return of U.S. prisoners.

He apologized in 2015 after referring to South Carolina U.S. Sen. Lindsey Graham, who's unmarried, as a "bro with no ho."

Kirk's latest comments drew heavy scorn across social media. Democratic presidential nominee Hillary Clinton posted on Twitter that she is thankful for Duckworth's and her family's service, adding "It's really not that hard to grasp, Mark Kirk."

The campaign manager for GOP presidential nominee Donald Trump also took the opportunity to jab at Kirk, who earlier this year withdrew his support for Trump and has been a vocal critic.

"The same Mark Kirk that unendorsed his party's presidential nominee and called him out in paid ads? Gotcha. Good luck," Kellyanne Conway posted on Twitter late Thursday.

Kirk, a first-term senator from Highland Park, is seen as one of the Senate's most vulnerable Republican incumbents, and Democrats consider Duckworth's success on Election Day one of the keys to reclaiming a majority in the chamber. Duckworth has a comfortable lead in the most recent polls, but Kirk says the race is closer than people think.